When a player dies, can he or can he not re-enter the game as a
new knight?
The rulebook have a section about new players, saying that
a newcomming player, can join the game as a new knight and
then draw 5 white cards.
But can a dead player do the same trick?

Unless you want to make home rules, the answer is no. In a certain way, it keeps up the challenge. If a dead knight could reenter the game, he would simply sacrifice all his life points to slow the progression of evil and make additional heroic actions. By the way, even if a knight is dead and the other loyal knights bring enough swords to the round table and win, he wins too

The rules don't allow you to simply die and then come back into the game; in fact they are very clear that if you die and nobody has the Grail you are out (if I remember right...I don't have the rules in front of me right now).

But if people really want to let people rotate back into the game because that's fun for them, I don't see why that couldn't be allowed by the group. In the end it's all about fun and as long as people are enjoying their game, they can play however they want!

maybe a little late to give my opinion but, I think you cant do that, because if you are traitor, and you dont like to be one, you could cheat game, die on purpose and take another knight with new loyalty card.